1. Field of the Invention
The present invention relates generally to inductive circuit elements, and more particularly to a controllable-inductance inductor or transformer architecture and a method of manufacturing the same.
2. Description of Related Technology
As is well known in the art, inductive components are electronic devices which provide the property of inductance (i.e., storage of energy in a magnetic field) within an alternating current circuit. Inductors are one well-known type of inductive device, and are formed typically using one or more coils or windings which may or may not be wrapped around a magnetically permeable core. So-called “dual winding” inductors utilize two windings wrapped around a common core.
Transformers are another type of inductive component that are used to transfer energy from one alternating current (AC) circuit to another by magnetic coupling. Generally, transformers are formed by winding two or more wires around a ferrous core. One wire acts as a primary winding and conductively couples energy to and from a first circuit. Another wire, also wound around the core so as to be magnetically coupled with the first wire, acts as a secondary winding and conductively couples energy to and from a second circuit. AC energy applied to the primary windings causes AC energy in the secondary windings and vice versa. A transformer may be used to transform between voltage magnitudes and current magnitudes, to create a phase shift, and to transform between impedance levels.
Ferrite-cored inductors and transformers are commonly used in modern broadband telecommunications circuits to include ISDN (integrated services digital network) transceivers, DSL (digital subscriber line) modems and cable modems. These devices provide any number of functions including shielding, control of longitudinal inductance (leakage), and impedance matching and safety isolation between broadband communication devices and the communication lines to which they are connected. Ferrite-core inductive device technology is driven by the need to provide miniaturization while at the same time meeting performance specifications set by chip-set manufactures and standards bodies such as the ITU-T. For example, in DSL modems, microminiature transformers are desired that can allow a DSL signal to pass through while introducing a minimal THD (total harmonic distortion) over the DSL signal bandwidth. As another example, dual-winding inductors can be used in telephone line filters to provide shielding and high longitudinal inductance (high leakage).
A common prior art ferrite-cored inductive device is known as the EP-core device. FIG. 1a illustrates a prior art EP transformer arrangement, and illustrates certain aspects of the manufacturing process therefor. The EP core of the device 100 of FIG. 1a is formed from two EP-core half-pieces 104, 106, each having a truncated semi-circular channel 108 formed therein and a center post element 110, each also being formed from a magnetically permeable material such as a ferrous compound. As shown in FIG. 1a, each of the EP-core half-pieces 104, 106 are mated to form an effectively continuous magnetically permeable “shell” around the windings 112a, 112b, the latter which are wound around a spool-shaped bobbin 109 which is received on the center post element 110. The precision gap in ground on the ferrite post 110 can be engineered to adjust the transfer function of the transformer to meet certain design requirements. When the EP core device is, the windings 112a, 112b wrapped around the bobbin 10 also become wrapped around the center post element 110. This causes magnetic flux to flow through the EP core pieces when an alternating current is applied to the windings. Once the device is assembled, the outer portion 21 of the EP cores 20 self-enclose the windings to provide a high degree of magnetic shielding. The ferrous material in the core is engineered to provide a given flux density over a specified frequency range and temperature range.
When completely assembled, the device 100 is mounted on top of a terminal array 114 generally with the windings 112a, 112b (i.e., the truncated portions 116 of the half-pieces 104, 106) being adjacent to the terminal array 114, which is subsequently mated to the printed circuit board (PCB) when the device 100 is surface mounted as shown in FIG. 1a. Note that the truncated portions are present, inter alia, to allow termination of the windings 112 outside of the device 100. Margin tape 117 may be applied atop the outer portions of the outer winding 112b for additional electrical separation if desired. FIG. 1b illustrates a surface mount implementation of the EP transformer mounted onto a circuit board.
Magnet wire is commonly used to wind transformers and inductive devices (such as inductors and transformers, including the aforementioned EP-type device). Magnet wire is made of copper or other conductive material coated by a thin polymer insulating film or a combination of polymer films such as polyurethane, polyester, polyimide (aka “Kapton™”), and the like. The thickness and the composition of the film coating determine the dielectric strength capability of the wire. Magnet wire in the range of 31 to 42 AWG is most commonly used in microelectronic transformer applications, although other sizes may be used in certain applications.
FIG. 1c illustrates a cross-section of the prior art device 100 after assembly.
Prior art EP inductive devices have several other shortcomings. A major difficulty with EP devices is the complexity of their manufacturing process, which gives rise to a higher cost. Also, the EP core half pieces themselves are relatively costly to mold and produce. For example, by the time the EP transformer is assembled and tested, its volume production cost is high (currently ranging from approximately $0.5;0 to −$0.70). It would be desirable to produce a device having performance characteristics at least equivalent to those of an EP transformer, but at a significantly lower cost.
The shielding of prior art EP core devices is also less than optimal, due ill large part to the shielding not being uniform around the device (i.e., magnetic flux permeating the “open” lower portion of the device).
Another disadvantage to prior art EP core inductors and transformers is the inability to individually control both the leakage inductance and the differential inductance of the transformer. The leakage inductance, also known as the common mode inductance, involves the inductive coupling loss between the transformer's windings. Control of the leakage inductance is important to many telecommunication applications. For example, the FCC imposes on-hook impedance limitations on circuits interfacing to telephone lines. The ETSI Specification requires a minimal “longitudinal impedance” (such as 10 KOhm) depending on frequency from each of tip and ring to ground. “Tip” and “ring” correspond to the two wires of a two-wire current loop as provided on a copper telephone wire. When designing with an EP transformers and inductors, in order to meet ETSI specifications, a second transformer is typically needed so that a pair of transformers is able to meet both a signal path transfer function requirement and a longitudinal inductance requirement. This can be very costly since the total DC resistance budget often requires all of the transformers to be larger to reduce the DC resistance of each transformer. The larger transformers are more expensive, physically consume more space, and have more parasitic capacitance. The increased parasitic capacitance results in lower bandwidth. It would be desirable to have a transformer that has a controllable common mode inductance so that the second transformer could be eliminated. This would provide smaller, less expensive transformer solutions that also have reduced parasitic capacitances and improved signal path frequency responses.
The main inductance of concern in a transformer is its differential inductance. The differential inductance is the inductance measured with the winding in series. While techniques to control the leakage inductance exist in EP transformers, an adjustment made to control of the differential inductance tends to have little effect on the leakage inductance. This lack of the ability to separately control the leakage inductances gives rise for the aforementioned need for two transformers to provide a transformer-system solution that meets both the signal path specification and the longitudinal inductance specification. It would be desirable to have a transformer architecture that could provide even partially independent control of both the differential and leakage inductances.
Based on the foregoing, it would be most desirable to provide an improved inductive component, related telecommunication circuits, and a method of manufacturing the improved inductive component. Such an improved device would involve a lower cost manufacturing process using inexpensive components to produce devices at a lower cost. It would also be advantageous if such a device could have independently controllable differential and leakage inductances to eliminate the need for a second device to control the leakage inductance as would otherwise be needed to satisfy a system-level longitudinal inductance specification. The elimination of the second device would further reduce costs at the system level and reduce the overall DC resistance, parasitic capacitance and footprint of the component. It would also be desirable for such an improved device to maintain desirable characteristics attributed to EP core devices such as small size, wideband performance, low THD, and also possess a high degree of electromagnetic shielding. Such an improved device could also be utilized in filter and splitter circuits to provide enhanced performance at lower cost that existing prior art solutions.